A commercially and technically important utility for biaxially oriented polyester films is to serve as a component of many articles such as food packaging, printing media, electrical insulation, optical and the other industrial uses. The thermal stability, dimensional stability, chemical resistance, relative high surface energy, optical clarity as well as cost effectiveness of biaxially oriented polyester films are beneficial for typical end use applications. Regarding optical clarity, biaxially oriented polyster films can be used for instance as a substrate of optical products such as window films, display parts, touch screen, eyewear, including visors, goggles, and spectacles, lenses, sunscreens, labels and photovoltaic materials. Typically applications will involve placing optically clear acrylic coating material onto the biaxially oriented polyester films.
Although, biaxially oriented polyester film and acrylic coating each are optically clear, the composite film of biaxially oriented polyester film coated with acrylic materal can have adverse optical properties. For example, the film can exhibit excessive iridescence. The refractive index (“RI”) of an acrylic coating material can be different from the RI of a biaxially oriented polyester film, e.g. polyethylene terephthalate (PET) film that is about 1.66. The difference between these refractive indices causes optical interference of light rays reflected from the surface between the acrylic layer and the polyester layer. This interference produces a rippled iridescent appearance through the spectral reflectance of the acrylic material-coated polyester film. Iridescence on the acrylic material coated polyester film is very evident under spectral light of fluorescent lamps because such light has a sharp distribution of luminescence that interferes with the rippled spectral reflectance of the acrylic material coated polyester film.
Iridescence is reduced or does not occur if the film is hazy because light is scattered. Hazy film is not desirable for many optically clear end use applications. Furthermore, the use of fluorescent lighting in place of incandescent lighting is increasing due to energy conservation efforts. Consequently, the iridescence of the acrylic material-coated polyester film can distort or block the view through the film or detract from the desired aesthetic appearance of the article comprising the film. Accordingly, the ability to reduce iridescence is gaining importance.
Another problem with the acrylic material-coated polyester films is adhesion between layers of acrylic material and the polyester layer. In general, biaxially oriented polyester film has a highly crystallized surface that makes the polyester difficult to adhere to an acrylic material coating layer. To overcome this drawback, a primer layer is sometimes used between polyester and acrylic polymer layers to improve adhesion.
Japanese Patent Publication Number JP 2004-299101 of YOKOTA SUNAO et al., entitled “Transparent Laminated Film for Surface Protection” is directed to a transparent composite film with a 10-250 μm thick base layer of biaxially stretched polyester and a 3-20 μm thick hard coat layer of acrylic polymer providing at least 90% light transmission. The film is for laminating to a surface of an article, such as a flat panel display member, a nameplate, a window and the like, to protect the article from scratching or other damage.
US patent application 2008/0038539 of Yokota et al. discloses a composite film having a core layer sandwiched between outer layers to form a base polyester layer. A coating layer containing anti-iridescent material covers one side of the base polyester layer and an acrylic coating is disposed over the anti-iridescent coating layer. US '539 discloses the effect achieving anti-iridescence by optimizing RI and the coating layer thickness to minimize infringement of reflection light which causes ripples of the spectral reflectance.
However, the products disclosed by these references do not fulfill the needs of modern industries for less or no iridescence, and for more robust adhesion under very harsh condition such as moisture exposure. The disclosed films above have been found to provide a hard coat with only moderate adhesion to the base layer, especially after exposure to heat and moisture.